make windows code built with clangcl include the _posix source code. depot_tools excludes these source files now, so they need to be manually included. BUG=435 TESTED=clangcl local build on windows R=tpsiaki@google.com
make windows code built with clangcl include the _posix source code.
depot_tools excludes these source files now, so they need to be manually
included.
BUG=435
TESTED=clangcl local build on windows
R=tpsiaki@google.com

Review URL: https://webrtc-codereview.appspot.com/52419004

git-svn-id: a51711482b6ba0e1d1ab095578a045eeae45ac9a@1396 16f28f9a-4ce2-e073-06de-1de4eb20be90

rig-project/libyuv,rig-project/libyuv,rig-project/libyuv